The he'patitiS'Bvirus (HBV) is a major cause of infectious liver disease throughout the world. There are 1.2 million carriers of HBV in the U.S. and approximately 400 million worldwide. Neonatal HBV infection is rarely cleared and as many as 90% of perinatally infected children become chronically infected. Therefore, in addition to worldwide vaccine programs to prevent new infections, methods for treating HBV chronic carriers will be necessary to eradicate this disease. This proposal is focused on understanding the mechanisms responsible for inducing and maintaining chronic HBV infection and more specifically the role of HBV nucleoprotein antigens, the nueleocapsid (HBcAg) and the secreted non-particulate HBeAg. Armed with this information we are designing immunotherapeutic vaccine candidates for the treatment of chronic hepatitis B infection. The specific aims are addressed through the use of HBc/HBeAg-expressing and HBV replicating tra'nsgehic(Tg) mice and three recently developed HBc/HBeAg-specific T cell receptor (TCR)-Tg lineages and the Various combinations of "double and triple-Tg" hybrids. The specific aims are: (1) determine mechanisms of HBeAg-induced t cell tolerance at the cellular and molecular levels; (2) examine the relationship between HBc/HBeAg-specific CD4+ and CD8+ T cells; (3) explore the reasons for "split tolerance" between the HBcAg and the HBeAg at the cellular and molecular levels; and (4) develop strategies to reverse or bypass HBeAg-induced T cell tolerance as possible immunotherapies for chronic HBV infection. During the previous years of this project we have developed a number of Tg model systems that will facilitate the pursuit of the current specific aims. For example, four in vivo models of HBeAg-induced T cell tolerance mediate'd by clohal deletion, clonal anergy, clonal ignorance and T regulatory (Treg) cells have been developed. Furthermore, models of in vivo anti-HBc and anti-HBe seroconversion and CD4+ T cell-mediated liver injury have been developed. These model systems should be very useful in elucidating the mechanisms of T cell tolerance that play an important role in promoting viral persistence during chronic HBV infection and in designing candidate vaccines for treating chronic HBV infection.